It has long been desired to develop alloys other than costly gold or other precious metals for use in the fabrication of metal tooth caps used as a base for porcelain application. For a period, nickel-chromium alloys containing small percentages of beryllium were used successfully; however, because of concern over allergic reactions to nickel and beryllium, such alloys are considered by some to be health hazards. As a matter of fact, certain countries have banned completely the use of these metals in dental alloys.
More recently, research has been undertaken to improve chromium-cobalt alloys, alloys which have been used for removable partial dentures since the 1930s, so that they could be used for porcelain application to form permanent dental structures. The generally used formulation of such alloys contains about 66% cobalt, 27% chromium, 5.5% molybdenum, 0.75% each of manganese and silicon, and 0.40% carbon. To date, however, such chromium-cobalt alloys suffer from several drawbacks making them unsuitable for the desired commercial applications. Firstly, the alloy expands too much during the process of baking porcelain to it, causing the porcelain to craze. These alloys have a minimum coefficient of expansion at 575.degree. C. of 14.1, an amount not quite within the limit of acceptability for baking porcelain to them. Secondly, chromium and cobalt oxides form on these alloys during baking, causing the porcelain to discolor. This enhances the likelihood of porcelain cracking and poor adhesion. Thirdly, the chromium-cobalt alloys are very hard, making finishing and polishing extremely difficult and labor-intensive. Finally, and also of significant importance, the chromium-cobalt alloys are higher fusing than nickel chromium and consequently exhibit considerable shrinkage after casting, resulting in poor-fitting crowns.
Several attempts have been made to overcome these deficiencies. For example, up to 10% tungsten has been added to the chromium-cobalt alloys to reduce the coefficient of expansion, and aluminum and columbium as well as rhenium and ruthenium have been used to reduce oxidation. While such alloys have resolved some of the problems, the fusing temperature of the resulting alloy is quite high which results in after-casting shrinkage and poor fit and hardness also remains a problem.